1. Field of the Invention
The present invention relates generally to optical transmitters and associated control systems. More particularly, embodiments of the present invention relate to systems and methods for enabling effective power and electrical modulation amplitude control of an optical transmitter so as to enhance the performance of an associated optical system and related components.
2. Related Technology
Many high speed voice and data transmission networks rely on optical transceivers and similar devices for facilitating binary transmission and reception of digital data embodied in the form of optical signals. Typically, data transmission in such networks is implemented by way of an optical transmitter, such as a laser, while data reception is generally implemented by way of an optical receiver, an example of which is a photodiode. In general, the optical transmitter and optical receiver communicate with each other by way of a suitable optical transmission medium, such as an optical fiber.
Due to the wide variety of applications wherein optical networks are employed, such optical networks may include, in addition to basic components such as optical transmitters and optical receivers, various other associated systems and devices that serve to implement one or more functionalities that are characteristic of a particular type of application. While such other systems and devices provide useful functionality, those systems and devices often implicate concerns relating to the overall performance of the associated optical system, as discussed below.
One type of optical communication system that implicates certain characteristic or unique problems are those systems sometime referred to as long-haul communication systems. Generally, long-haul communication systems refer to those optical systems, and associated devices, that are used to facilitate voice and/or data transmission over relatively long geographical distances. One example of a long-haul communication system is a fiber optic telephone network. While long-haul, fiber optic communication systems are useful in transmitting high volumes of voice and data over long distances, such long transmission distances implicate certain requirements and associated problems with respect to the structure and operation of the long-haul communication system.
By way of example, the quality of optical signals that are transmitted over relatively long distances tends to degrade over distance. One approach to addressing this problem involved the use of repeaters located at predetermined intervals along the transmission path. In general, the repeater receives an optical signal, converts the optical signal to an electrical signal which is then amplified by the repeater. After amplification, the repeater then converts the amplified electrical signal back into an optical signal and transmits the optical signal to the next repeater or end node. Thus, such repeaters require that operations be performed both in the electrical and optical domains.
While repeaters proved useful in some applications, the necessity of operating in both electrical and optical domains is problematic because repeaters can only operate on a single data stream or optical channel. For example, wavelength multiplexed systems have many data streams on many separate optical channels, where each channel corresponds to a different wavelength. Thus, each data stream would have to be demultiplexed, presented to a multiplicity of repeaters, then again multiplexed back into a single fiber. Not only is this process operationally complex, but signal losses are incurred both during demultiplexing and subsequent multiplexing.
In light of the inherent limitations of repeaters, “pure” optical amplifiers have been developed that operate strictly in the optical domain and thus are well-suited for wavelength multiplexed applications. Generally, long-haul communication systems employ a number of optical amplifiers which are located at predetermined intervals along the transmission path and serve to amplify the transmitted optical signals so that a high quality signal is received at even the most remote node, or receiver, of the optical network.
One example of such an optical amplifier is an erbium-doped fiber amplifier (“EDFA”). Optical amplifiers such as EDFAs have been widely employed as a result of their ability to produce an accurately amplified replication of any complex optical input, possibly containing many data streams at different optical wavelengths, for further transmission along the network. Consequently, optical amplifiers have proven particularly popular in wavelength division multiplexing (“WDM”) applications.
In general, optical amplifiers such as EDFAs include a length of doped optical fiber. Typically, the doping element is a rare earth element, such as erbium, having an atomic structure such that the doped fiber is suitable for amplifying light. The EDFA operates in connection with one or more pump lasers that serve, in general, to exploit the light amplification ability of the atomic structure of the doped fiber.
More particularly, such pump lasers typically operate at a wavelength of 980 nanometers (“nm”) or 1480 nm and operate to introduce energy into the doped fiber. As the degraded optical signal, typically having a wavelength of 1310 nm or 1550 nm, enters the EDFA, the energy generated by the pump laser serves to excite the erbium or other rare earth element in the doped fiber so that the erbium atoms release their stored energy in the form of additional 1550 nm or 1310 nm light. This process is repeated as the optical signal passes through the doped fiber, so that by the time the optical signal reaches the end of the doped fiber, the optical signal is of sufficient strength to be re-transmitted for an additional distance.
As suggested by the foregoing, optical amplifiers provide useful functionality in a long-haul optical networks. However, the performance of such optical amplifiers and, thus, the performance of the optical network as a whole, is often adversely affected by the systems, methods and devices used to control the operation of the optical transmitter.
For example, optical amplifiers such as EDFAs are particularly sensitive to rapid changes in the optical power at their inputs resulting from the “on” and “off” switching of the transmitters which feed data to the EDFA. As discussed below, such sensitivity can cause problems with the optical receivers of the system.
With respect first to the laser bias to the optical transmitter, typical transmitter control systems are configured so that laser bias to the optical transmitter is abruptly, rather than gradually, applied when it is desired to enable operation of the optical transmitter. In similar fashion, typical transmitter control systems abruptly cut laser bias when it is desired to disable operation of the optical transmitter. These abrupt changes in laser bias cause correspondingly rapid changes in the level of optical power received by the optical amplifier, thereby impairing the operation of the controller of the optical amplifier.
Moreover, because typical optical amplifiers are unable to respond quickly to sudden changes in input optical power, the optical output of the amplifier changes for a period of time as the controller of the optical amplifier attempts to achieve and maintain a constant output power. This fluctuation of the output optical power results in fluctuations in the optical output power of all other signals passing through the amplifier, thereby causing problems in their respective associated receivers, such as the receipt, at the receivers, of incorrect and/or incomplete data. In other cases, the receivers are simply unable to process the received signal for a period of time until the amplifier output restabilizes, or the respective receivers have time to adapt to the new optical signal strength presented to them.
As noted above, another problem concerning typical optical transmitter control systems relates to changes in the electrical modulation amplitude of the optical signal transmitted onto the system by the optical transmitter. In general, binary data transmission by an optical transmitter of an optical system is accomplished by transmitting at a relatively higher optical power to indicate a binary “1,” and transmitting at a relatively lower optical power to indicate a binary “0.” The ratio of the “1” power level to the “0” power level is sometimes referred to as the “extinction ratio,” or simply, ER.
Because optical receivers are primarily responsive or receptive to the extinction ratio between the “1” and “0” optical power levels, and not to the average optical power, so long as the average power is above some minimum value and below some maximum value, it is desirable to maintain the extinction ratio at a relatively high, and constant, level. Accordingly, as the extinction ratio drops to a certain point, the optical receiver becomes increasingly unable to accurately or completely detect the incoming data. On the other hand, when the extinction ratio is maintained at a relatively high value, the optical receiver can readily detect the incoming data over a wider range of average optical power levels.
Typical optical systems, and optical transmitters and control systems in particular, are configured so that the electrical modulation signal amplitude which creates the optical data signal is maintained at a high level while power to the optical transmitter is shut down in response to a transmitter disable signal. This approach to data signal modulation control has proven problematic however. In particular, maintenance of the electrical modulation amplitude during laser shutdown often results in undesirable optical spikes being transmitted to the receivers as the modulation peaks exceed the threshold of the otherwise unbiased laser. Because the laser is unbiased, such optical peaks compromise the desired “quiet” state of the data channel.
Various other problems associated with known optical transmitter control systems, devices and methods concern the occurrence of undesirable optical signal wavelength effects, and the response of the receiver to the optical signal received from the optical transmitter. By way of example, uncontrolled, or ineffective control of, electrical modulation amplitude results in undesirable wavelength chirp, or the shifting of the center wavelength, which may adversely affect other data streams on adjacent optical channels. Further, the aforementioned concern with respect to electrical modulation amplitude also results in asymmetrical eye patterns, discussed in further detail below, and/or random spikes that compromise the ability of the optical receiver to receive and process the transmitted optical signal. This is particularly problematic because the disruption of the EDFA output power caused by the output power change of one optical transmitter will affect all other data streams on all other channels in that EDFA.
In view of the foregoing, and other problems, it has become apparent that solutions are needed to eliminate, or at least attenuate, such problems and shortcomings concerning optical transmitter input bias and optical signal modulation. One possible approach would be to employ an optical transmitter control method where, at startup of the optical transmitter, the electrical modulation amplitude of the optical signal, as well as the laser bias input power to the optical transmitter, are increased in tandem, each at a substantially constant rate, up to respective setpoints. Such a method would involve substantially the reverse process upon transmitter disable. Specifically, the modulation amplitude of the optical signal, as well as the laser bias input power to the optical transmitter, are decreased in tandem, each at a substantially constant rate, down to respective minimums.
While this type of approach would seem to represent somewhat of an improvement over previous methods and systems that involve abrupt changes in laser bias and/or electrical modulation amplitude, such an approach nonetheless produces undesirable “eye” patterns as the power and electrical modulation amplitude move upward from a zero point or move downward to a zero point. More particularly, the eye pattern refers to the waveform of a signal and is typically plotted as the amplitude of the signal over a predetermined period of time, where the 0-to-1 transitions are superimposed on the 1-to-0 transitions to show the relative difference between the two states and to show the rates of change between the two states.
In general, the openness of the eye pattern indicates the relative quality of the optical signal, so that a relatively more open eye pattern implicates a relatively higher quality optical signal, while a relatively closed eye pattern indicates a problem with the optical signal such as noise or bandwidth problems. Thus, the partially “closed,” asymmetrical, or otherwise impaired, eye patterns produced by typical optical systems as the laser bias and electrical modulation amplitude move upward from a zero point during transmitter enable, or move downward to a zero point during transmitter disable, reflect undesirable transmitter performance.
In view of the foregoing, it would be useful to provide systems and devices for implementing a method for controlling laser bias to an optical transmitter, as well as for controlling the electrical modulation amplitude of an optical transmitter. Among other things, the method should enable variable rates of change to both the laser bias and to the electrical modulation amplitude of the transmitter, and should also enable the electrical modulation amplitude and laser bias to be adjusted in a predetermined time relationship with respect to each other. Finally, the method should enable changes in electrical modulation amplitude and laser bias to be triggered by the occurrence of predefined events such as optical transmitter enable and disable events.